1. Field of the Invention
The present invention relates to the removal of hydrogen sulfide from gas streams with an aqueous washing solution and, more particularly, to a method for increasing the size of sulfur particles which are removed from the washing solutions of hydrogen sulfide removal processes wherein absorbed hydrogen sulfide is converted to elemental sulfur.
2. Description of the Prior Art
With the increasing concern over atmospheric pollution and the concomitant increasingly strict enforcement of ever more stringent air pollution standards, greater responsibility is being placed on industry to produce pollution-free products in a non-polluting manner. One area of particular concern in the past, and still today, is the discharge or release of sulfur and its compounds, especially hydrogen sulfide (H.sub.2 S), into the atmosphere as a result of a number of industrial processes. These processes include, for example, petroleum refining, the roasting or smelting of various sulfide containing ores, the sweetening of sour natural gas, destructive distillation of coal and oil shale, gasification or liquefaction of coal, and the production and use of H.sub.2 S-containing geothermal steam and liquid for generating electricity and for other uses.
Several, processes have been developed, and are in relatively common use, for removing H.sub.2 S from gas streams such as those generated or encountered in the industrial processes listed above. One such hydrogen sulfide removal process is the Stretford process, which employs an aqueous, alkaline washing solution to preferentially absorb and oxidize H.sub.2 S therein. Absorption is done with a water-soluble organic alkaline agent, such as anthraquinone disulphonic acid (ADA), with the hydrogen sulfide being oxidized to particles of elemental sulfur by a pentavalent vanadium compound such as sodium vanadate (NaVO.sub.3) Recovery of the sulfur is accomplished by flotation, using a stream of air which is injected into the process solution. This generates a frothy slurry containing the sulfur particles which rises to the top of the solution where it is skimmed off, with the sulfur therein being recovered therefrom by filtration or other liquid/solid separation techniques. In this process, the oxygen in the injected air also serves to reoxidize the reduced vanadae ions and thus regenerate the solution for reuse in the process.
There are a number of other processes commercially available to perform such sulfur oxidation and recovery. One of these is the Unisulf process, which is described by Fenton et al. in U.S. Pat. No. 4,283,379, the disclosure of which is incorporated herein by reference in its entirety. In this process, the washing solution comprises a solubilized vanadium salt as the oxidizer, a no-quinone aromatic absorption compound, thiocyanate ions, and a water-soluble carboxylate complexing agent. Other processes are based on the use of other metallic oxidizers such as ferric iron and soluble arsenates.
A commonly used technique to remove the sulfur particles is by circulating the washing solution through a tank-like oxidizer vessel, through which air is bubbled to regenerate said washing solution and form said frothy slurry. With fresh, unused solutions, the elemental sulfur particles which, when formed, have an average diameter in the range between about 0.5 and 5.0 microns, agglomerate to form sulfur clumps of about 10 to about 150 microns in size. Particles of this size are readily buoyed up to the surface in the aforementioned froth and pass through a weir-like opening near the top of the vessel into a sulfur collection vessel. Here, the bubbles in the froth readily collapse, and the resultant liquid suspension or slurry can be easily pumped to a sulfur separation device such as roary vacuum filter, filter press or centrifuge, from which, after washing to remove the entrained process solution, an extremely pure grade sulfur is obtained. Where a nonpparticulate form of sulfur is desired, the washed filter cake may be sent to an autoclave or other sulfur melter.
One problem which has plagued the operators of all such processes is that of efficiently removing the elemental sulfur particles from the circulating washing solution. Inefficient sulfur particle removal causes an unwanted buildup in the concentration of such particles within the washing solution, which, in turn, raises the total mass of the suspension being circulated, thus increasing pumping expenses. Secondly, as the sulfur concentration increases, it is found that some of the particles drop out of suspension, particularly in areas of high centrifugal force such as sharp bends in the associated plumbing. The resultant formation of deposits can cause partial and, in extreme cases, total plugging of the circulation system, thereby further increasing pumping expenses and often forcing a complete, and usually unschedule, shutdown of operations. In addition, large quantities of circulating sulfur particles will eventually cause other significant problems due to both corrosion and erosion in the circulation piping, pumps and other components of the plant acility. The presence of these particles also acerbates a tendency of older solutions to foam. Foams, which have high liquid-to-air ratios and generally very small bubble sizes, as compared to froths, which have larger bubbles and a relatively low liquid content, are very poor vehicles for transferring sulfur out of the oxidizer vessel. As a result, foams leave an inordinate amount of small particles circulating with the washing solution. Lastly, circulating sulfur particles in high concentrations can react with various washing solution constituents and accelerate the normal buildup of nonregenerable contaminants such as sodium thiosulfate and sodium sulfate. When the levels of such contaminants reach sufficiently high levels, the vanadium-containing solution becomes unusable and usually must be disposed of by deep well injection, or by sending it to a hazardous waste landfill, with resulting high costs and potential liability for future cleanups.
It is known that, after the washing solution has been in use for some length of time, the overall effectiveness of sulfur removal by the above described technique begins to degrade. This is believed to result from the normal buildup of sulfates and thiosulfates along with contaminant salts such as colloidal metal oxysulfides and, particularly in the Stretford process, ADA oxidation products. These materials are believed to inhibit the agglomeration of the initially formed small sulfur particles into larger clumps as herein above described. Small particles are not easily floated up to the surface by bubbled air and thus have a greater tendency to stay suspended in the circulating washing solution than do larger sized particles, with all the problems noted above.
Even when they are floated out of the system in the froth, these small (i.e., under 5 microns in diameter) particles frequently require a considerable amount of wash water to completely remove the last traces of washing solution from them. This situation occurs because sulfur particles are normally hydrophilic and, therefore, readily wetted by the washing solution. As a result, some amount of solution is adsorbed onto the surfaces of these particles and carried out with them. This loss is in addition to that resulting from removing the froth. Where, for same reason, the aforesaid agglomeration has, in whole or in part, failed to occur, the relatively large surface-to-volume ratio of these unagglomerated particles will cause the amount of solution so adsorbed and carried out to be quite large. In extreme cases, the amount of washing solution carried out is large enough that the resultant sulfur is too contaminated for most, if not all, commercial use and, thrrefore, must, under current EPA regulations, be discarded at a hazardous waste dump.
There is therefore a need for a technique for increasing the size of sulfur particles produced in thiosulfate and sulfate salt contaminated, regenerable aqueous washing solutions used in processes wherein absorbed hydrogen sulfide is converted to elemental sulfur. In the present invention, such a technique has been found.